As Linux fans know, there are two kinds of hackers: the good guys who develop free software, such as the Linux kernel, and the bad guys who break into computers.

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As Linux fans know, there are two kinds of hackers: the good guys who develop free software, such as the Linux kernel, and the bad guys who break into computers.

The bad guys paid the good guys an unwelcome visit earlier this month, breaking into the Kernel.org website that is home to the Linux project. They gained root access to a server known as Hera and ultimately compromised "a number of servers in the kernel.org infrastructure," according to a note on the kernel.org website Wednesday.

Administrators of the website learned of the problem Sunday and soon discovered a number of bad things were happening on their servers. Files were modified, a malicious program was added to the server's startup scripts and some user data was logged.

Kernel.org's owners have contacted law enforcement in the U.S. and Europe and are in the process of reinstalling the site's infrastructure and figuring out what happened.

They think that the hackers may have stolen a user's login credentials to break into the system, and the site is making each of its 448 users change their passwords and SSH (Secure Shell) keys.

The hack is worrying because Kernel.org is the place where Linux distributors download the source code for the widely used operating system's kernel. But Kernel.org's note says that, even with root access, it would be difficult for a hacker to slip malicious source code into the Linux kernel without it being noticed. That's because Linux's change-tracking system takes a cryptographic hash of each file at the time it is published.

So once a component of the Linux kernel has been written and published to Kernel.org, "it is not possible to change the old versions without it being noticed," the Kernel.org note said.

This kind of compromise has become disturbingly common. In January, servers used by the Fedora project -- the community version of Red Hat Enterprise Linux -- were hacked. And around the same time another open-source software development site called SourceForge was also broken into.